The present invention is related to methods and structures for improving mechanical strength in Micro Electro Mechanical Systems (MEMS). These methods and structures are used to produce MEMS devices with improved mechanical characteristics.
Micro Electro Mechanical Systems (MEMS), also known as Microsystems or Micro Machined Systems, use the process technology as developed in semiconductor processing to obtain devices with the desired mechanical properties. In MEMS technology, these mechanical devices on microscale are embedded in micro-electronic circuitry. MEMS devices serve as an intermediate between the non-electrical and the electrical world or as transducers between physical quantities (e.g., transforming radiation or pressure into current). Contrary to semiconductor devices, mechanical properties such as weight, vibration, etc. are crucial for MEMS devices. The characteristics of Micro Electro Mechanical Systems (MEMS), e.g. accelerometers, thermal sensors, membrane-devices etc., are thus determined by both their transducing and mechanical properties. The strong interaction between the transducing and mechanical properties of these systems often imposes restrictions on meeting simultaneously both types of specifications.
Examples of Micro Electro Mechanical Systems devices are thermal sensors. Thermal sensors measure a temperature rise resulting from the deposition of an amount of energy within a thermally insulated piece of material, this insulated piece being the sensing element. The energy can result from a variety of interactions between the environment and the sensing element, e.g. X-ray or infrared flux absorption, reacting molecules. A xe2x80x9cMethod of fabrication of an infrared radiation detector and more particularly an infrared sensitive bolometerxe2x80x9d is disclosed in U.S. Pat. No. 6,194,722 and is entitled xe2x80x9cMethod Of Fabrication Of An Infrared Radiation Detector And Infrared Detector Device.xe2x80x9d U.S. Pat. No. 6,194,722 is hereby incorporated by reference in its entirety.
Thermal analysis shows that the change in temperature is proportional to the thermal insulation of the device layer, the energy deposited in this layer being in general a given quantity. The better the thermal insulation of the sensing element, the more sensitive the device becomes. The energy, deposited in the sensing element, will not or to a less degree, leak away to the environment thanks to the better insulation. A larger change in temperature is thus obtained for a given amount of incident energy. Maximum thermal insulation can be obtained by suspending the sensing element on thin narrow and long beams of a thermally insulating material, so that the sensing layer is only connected to the rest of the device by these suspending beams. Between the sensing element and underlying layer there is a gap providing better thermal insulation of the sensing element.
The very last process step, being the removal of the sacrificial layer between the sensing element and the underlying substrate thereby creating the aforementioned gap, is critical and difficult. This removal is generally done by a wet etch. Due to a general problem in surface micromachining, called sticking, the removal of such a sacrificial layer causes a lot of yield loss. During the wet etch of the sacrificial layer, the surface tension of droplets under the sensing element pulls this sensing element towards the substrate. Once the sensing element reaches the substrate, strong bonding forces between this sensing element and the substrate along with the mechanical weakness of the supporting beams prevent the sensing element from returning to its original position. Once the sensing element touches the substrate, the device has a large thermal conductance to the substrate and is useless as a bolometer. Making the supporting beams thinner will improve the thermal insulation of the sensing element but turns out to make the problem of sticking more severe, resulting in yield loss during processing and packaging of such devices.
In xe2x80x9cMechanical performance of an integrated microgimbal/microactuator for disk drives,xe2x80x9d Proc. of the Transducers ""99 Conference, June 1999, Sendai, Japan, p. 1002-1005 (1999) by L. Muller, J. M. Noworolsky, R. T. Howe and A. P. Pisano, a specific structure is presented to improve the mechanical strength of parts of an assembled device, i.e., a device for reading data stored in a disk drive. However, this structure is obtained at the expense of additional, time consuming, process steps. These process steps cannot be done in the course of producing the gimbal, but require separate processing, followed by assembly. The proposed process uses thick layers having large dimensions up to 75 micrometer in this so-called high-aspect-ratio fabrication technology. Only the torsional stiffness of the supporting bars is improved, no other mechanical or physical characteristics of the device are involved by the processing of the microgimbal.
The article xe2x80x9cSilicon micro/nanomechanical device fabrication based on focused ion beam etching surface modification and KOH ethcing,xe2x80x9d in Microelectronic engineering 35 (1997) p 401-404, by J. Brugger et al. discloses the improvement of the mechanical stability of freestanding nanomechanical elements by increasing the moment of inertia of the supporing elements. The technique disclosed is very complex and time consuming and does not allow waferscale processing.
One aspect the present invention aims to improve the mechanical stability of the MEMS devices. The improved mechanical strength can lead to higher production and packaging yield. It can also improve the operation of MEMS devices that have moving elements. By introducing this method, the thickness of layers can be adapted to meet other technological specifications and still have sufficient mechanical strength. The improved mechanical strength can lead to higher production and packaging yield. The improvement in mechanical strength with respect to the device properties can be expressed in terms of a figure of merit M. In the finished MEMS device, layers may indeed not be fully supported by an underlying layer and hence lack mechanical strength.
Another aim of the present invention is to provide a device for sensing electromagnetic radiation with improved mechanical characteristics and device performance.
Another aim of the present invention is to obtain an infrared sensor with improved mechanical characteristics and device performance. An advantage of this device is that the thickness of the layer, e.g. the sensing layer, can be adapted to have the maximum temperature sensitivity thanks to a minimal thermal conductance possible for a given layout or material choice. Another advantage of such a device is that the time constant of this thermal sensor is minimized to such a level that the device can be applied in fast thermal sensor camera applications. The improved mechanical strength allows using a minimal layout of the devices so the thermal sensor can be used in an array-type of circuitry. The improvement in mechanical strength with respect to the device properties can be expressed in terms of a figure of merit M. In the present application, the term xe2x80x9clayerxe2x80x9d should be understood as a stack of at least one layer. U- or I-shaped layer means that a part of the cross section of this layer has a U- or I-shaped profile. Generally, a structure in a MEMS device which provides a three-dimensional shape to a part of a MEMS device will be called a xe2x80x9cmicrostructurexe2x80x9d and insofar as it improves the mechanical properties of the device, e.g. its rigidity, it will be called a xe2x80x9crigidizing microstructure.xe2x80x9d In one aspect of the invention, MEMS devices with improved mechanical characteristics are provided.
The aims of the invention are solved by devices and methods defined in the attached claims.
In one embodiment of the invention, the resistance of layers in MEMS devices to bending, lateral or torsion forces, having a static or dynamic nature, is increased by providing these layers with one or more rigidizing microstructures. The term xe2x80x9crigidizedxe2x80x9d is well known in the mechanical arts and refers to providing structures such as corrugations to improve the mechanical properties of a sheet material, especially improving the resistance to bending and/or deformation. xe2x80x9cInternally rigidizedxe2x80x9d refers to a form of rigidizing in which the reinforcing structures are contained within the confines of a layer, i.e., it does not involve the application of external rigidizing elements which extend beyond the edges of the layer concerned. Preferred microstructures are those similar to an I- or U-profile or a combination of such profiles in a two-dimensional array instead of using the conventional rectangular profiles. In the finished MEMS device, layers may indeed not be fully supported by an underlying layer and hence lack mechanical strength. At least a part of non-fully supported layer is a membrane, that is a thin layer of such an extent that it has a recognizable and useful area. Such a membrane may be come in a variety of shapes including circular, elliptical, oval, quadratic, a parallelepiped or a similar 2-D shape overall. The membrane is attached to a substrate by a support system.
The present invention provides a device comprising a membrane element supported by a supporting structure attached to a substrate and usually to one or more points are areas on one or more edges of the membrane element. The supporting structure may be beams, anchor points or areas, or similar structures. The connection point of a beam to the membrane or the anchor portions of the membrane may be substantially at the perimeter of the membrane, but the present invention is not limited thereto.
The support system such as a beam may have a layer which is common with the membrane element. This common layer may be deposited and processed at the same time. This allows processes for rigidizing the membrane element and the support structure to be carried out at the same time involving introduction of reinforcing microstructures into these elements, or alternatively only into the support structure or only into the membrane element to be rigidized.
In another embodiment of the invention a method is presented to obtain 3-D microstructures such as I- or U-shaped profiled layers by applying a sequence comprising processing steps, such as depositing and patterning of layers, steps known themselves as a common practice specifically in MEMS process technology and more generally in semiconductor process technology but without the novel features of the present invention.
In another embodiment of the invention a MEMS device is disclosed comprising a planar element or membrane, having an improved mechanical strength.
In another aspect of the invention a device is presented containing at least one membrane with a rigidizing microstructure, for example, an I or U-shaped profiled layer.
For example, in another embodiment of the invention these microstructures, e.g. I- and U-profiles, can be repeated along and/or across the layer. The profiles may be combined within the layer, especially to provide a rigidizing grid of elongated microstructure elements extending in two dimensions.
In another aspect of the invention, a thermal sensor, e.g. infra-red sensor, having a membrane, especially a membrane which supports or is integral with a sensing element of the sensor, is provided having improved mechanical strength to bending or lateral forces, by applying a microstructure in the membrane e.g. an I or U-shaped layer or a combination thereof. whereby The improved mechanical strength of the planar element allows the use of thinner layers and results in a faster read-out of the sensor.